Naturally occurring mutations at the acetylcholine receptor binding site independently alter ACh binding and channel gating

By defining functional defects in a congenital myasthenic syndrome (CMS), we show that two mutant residues, located in a binding site region of the acetylcholine receptor (AChR) epsilon subunit, exert opposite effects on ACh binding and suppress channel gating. Single channel kinetic analysis reveals that the first mutation, epsilonN182Y, increases ACh affinity for receptors in the resting closed state, which promotes sequential occupancy of the binding sites and discloses rate constants for ACh occupancy of the nonmutant alphadelta site. Studies of the analogous mutation in the delta subunit, deltaN187Y, disclose rate constants for ACh occupancy of the nonmutant alphaepsilon site. The second CMS mutation, epsilonD175N, reduces ACh affinity for receptors in the resting closed state; occupancy of the mutant site still promotes gating because a large difference in affinity is maintained between closed and open states. epsilonD175N impairs overall gating, however, through an effect independent of ACh occupancy. When mapped on a structural model of the AChR binding site, epsilonN182Y localizes to the interface with the a subunit, and epsilonD175 to the entrance of the ACh binding cavity. Both epsilonN182Y and epsilonD175 show state specificity in affecting closed relative to desensitized state affinities, suggesting that the protein chain harboring epsilonN182 and epsilonD175 rearranges in the course of receptor desensitization. The overall results show that key residues at the ACh binding site differentially stabilize the agonist bound to closed, open and desensitized states, and provide a set point for gating of the channel.